I think we have to take into account the big disconnect between the way trap was shot a hundred years ago and the way it is today.
Yesterday's ammo was inferior - softer shot, no shot wrappers, and I suspect the payload was slower. To add some parity, ounce and a quarter loads were legal. Targets were faster, lower, wider, and not as stable in flight. All of this would have favored a full-choked barrel(s).
Judging from the breech faces of the three SxS trapguns I own, it looks as if both barrels were used more or less equally, which I suspect was not because of all the double targets that were shot, but rather a deliberate act on the part of the shooter in order to distribute the work.
Back to the chokes. Of the pre-war double trap guns I have seen, only one had something other than F&F or M&F, and that was 0.013" and 0.027", making it an obvious dedicated doubles gun for a fast shooter. However, I don't think those chokes would have been feasible back in the day before shot collars, and my suspicion is that the barrels were opened long after the gun was made.
Do doubles work at the traps? Hall of Famers John Sternberger and Hiram Bradley were using M21s through the sixties and into the early seventies. Maybe they didn't know any better.
